The present invention relates to electric motors and more particularly, the present invention relates to electric motors especially suited for use in the borehole drilling art. Motors of this type must be such as to meet the special space requirements of a borehole, so that the outside diameter is generally very limited, whereas such motors may be very long. The precise length depends on the desired power of the motor. Further to the special space requirements during operations in a downhole environment, this type of environment also represents challenging conditions such as high pressure, high temperature and an acidic environment.
One difficulty encountered in motors of this type is that the rotor shaft must be tightly supported by at least bearings in each end of the motor, and typically also bearings in intermediate positions to ensure that the rotation of the rotor is progressing without wobbling, which would induce increased tear of the electrical motor and decreased power output. Therefore, a very tight fit of the rotor within the stator is essential in the design of an efficient electrical motor. Thermally induced thrust loads on the rotor present a challenge to the design of such electrical motors, primarily since, due to thermal expansion of the rotor, the rotor may bend towards the stator resulting in increased friction and a wobbling motion of the rotor.
Prior efforts to circumvent this problem have been made, such as U.S. Pat. No. 3,136,905 disclosing a hollow rotor for allowing cooling water inside the rotor, thereby avoiding large thermal expansions of the rotor. By using this approach, another problem is introduced in the design, as it becomes important to provide a good seal on a rotating member, which is a difficult task under normal up-hole operating conditions as it is, but is considerably more complicated when working downhole with increased temperatures and increased pressure differences.
Therefore, a need exists for providing an improved electrical motor with the ability to maintain a tight fit of the rotor inside the stator and maintain the tight fit during thermal expansions during downhole operations.